a. Field of the Invention
Broadly speaking, this invention relates to radio transmission. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for eliminating crosstalk in radio systems of the type that employ single-sideband modulation.
B. Discussion of the Prior Art
As is well known, a considerable portion of the Nation's long-haul telecommunications traffic is carried over a coast-to-coast network of high-capacity microwave radio systems. This network is characterized by a large number of junctions or towers where two or more different routes cross. Foreground reflections generally limit antenna discrimination on such crossing routes to less than 70 dB which leads to interference between routes using the same channel assignments. This type of interference is called co-channel interference because the interfering signal occupies the same frequency spectrum as does the desired signal. There are, of course, other mechanisms producing co-channel interference but the effect is the same.
Heretofore, co-channel intereference has not been particularly annoying to the average subscriber because virtually all of these microwave systems employ frequency modulation and frequency modulation exhibits substantial resistance to this type of interference, due to the high modulation index placed on an FM signal.
More specifically, in an FM system the high index components, generally called "burble", are located well below the message band and their effect is to spread or smear the message load in the frequency domain. Thus, if an interfacing, co-channel signal enters an FM receiver where either the interference and/or the desired signal are burbled, the interference remains smeared but the desired signal is properly demodulated. The smearing of an FM signal has two basic effects; first, it reduces the amplitude of the signal in each interfering voice circuit; and second, it mixes several interfering circuits so that they appear together at one desired location in the receiver. The net result is that, on the average, the interference power into a desired circuit is the same as without burble but the noise is now close to thermal in nature and consequently, much less annoying.
Although frequency modulation systems are widespread and highly successful, the ever-increasing demand for higher system capacity, together with a shortage of available frequencies in the microwave spectrum, has led to a search for a more efficient modulation scheme. This search has concentrated on single-sideband modulation (SSB), a well-known technique wherein the carrier frequency and one of the two AM sidebands centered about the carrier frequency are suppressed prior to transmission. Unfortunately, with the use of SSB modulation co-channel interference can no longer be so easily dismissed. This is due to the high frequency stability inherent in single-sideband modulation. This high degree of stability causes an interfering SSB channel to produce intelligible crosstalk in the interfered-with system, and this crosstalk is subjectively far more disturbing to a subscriber than thermal noise would be.
A slight misalignment of the carrier frequencies used to generate the baseband voice channels, for example by a few hundred Hz, might conceivably be used to render the voice interference unintelligible and so less annoying, but tone interference, for example, from multifrequency operator or customer dialing tones, or from supervisory signals transmitted on the voice channels, would remain a problem.